Made of a dichroic glass, the famous Lycurgus cup created by the Romans in 400 A.D. exhibits different colors depending on whether light is passing through it; it shines red when lit from behind and green when lit from the front. The cup is also the impetus for all contemporary nanoplasmonics research — the study of optical phenomena in the nanoscale vicinity of metal surfaces.

Now, the Roman cup has inspired University of Illinois at Urbana-Champaign researchers to develop a nanoplasmonic spectroscopy sensing device that, for the first time, achieves colorimetric sensing.

The madness of King Lycurgus; side A of a late Roman cage cup in dichroic (changing color) glass, here glowing red because light is shining through it from behind. When lit from the front, the cup appears green. Purchased with the assistance of the National Art Collections Fund. Photographer: Marie-Lan Nguyen.
“It can be used for chemical imaging, biomolecular imaging and integration to portable microfluidics devices for lab-on-chip applications,” said Logan Liu, an assistant professor of electrical and computer engineering and of bioengineering. His research team’s results were published in the inaugural edition of Advanced Optical Materials (doi: 10.1002/adom.201370001).

Liu and colleagues achieved the dichroic effect in their device by including tiny proportions of minutely ground gold and silver dust in the glass.

“In our research, we have created a large-area, high-density array of a nanoscale Lycurgus cup using a transparent plastic substrate to achieve colorimetric sensing,” Liu said. The sensor consists of about 1 billion nanocups in an array with a subwavelength opening and decorated with metal nanoparticles on side walls, having a similar shape and properties as the Lycurgus cups displayed in a British museum. The investigators were particularly excited about the extraordinary characteristics of the material, which is 100 times more sensitive than any other reported nanoplasmonic device.

Colorimetric techniques are attractive because of their low cost, inexpensive equipment, need for less signal transduction hardware and, most importantly, for their ability to provide simple-to-understand results. Colorimetric sensors can be used for qualitative analytic identification and quantitative analysis; the current design will enable new technology to be developed in the field of DNA/protein microarray.

A model of nanocup arrays developed at the University of Illinois at Urbana-Champaign for chemical, DNA and protein analysis. The ultrasensitive tool was inspired by optical characteristics first demonstrated by the ancient Romans. Courtesy of University of Illinois at Urbana-Champaign.
“Our label-free colorimetric sensor eliminates the need of problematic fluorescence tagging of DNA/protein molecules, and the hybridization of probe and target molecule is detected from the color change of the sensor,” said first author Manas Gartia. “Our current sensor requires just a light source and a camera to complete the DNA sensing process. This opens the possibility for developing affordable, simple and sensitive mobile-phone-based DNA microarray detector in near future. Due to its low cost, simplicity in design and high sensitivity, we envisage the extensive use of the device for DNA microarrays, therapeutic antibody screening for drug discovery, and pathogen detection in resource-poor setting.”

Light-matter interaction using subwavelength-hole arrays gives rise to interesting optical phenomena such as surface plasmon polaritons (SPPs) mediated enhanced optical transmission (EOT), Gartia said. In the case of EOT, more than the expected amount of light can be transmitted through nanoholes on otherwise opaque metal thin films. Since the thin metal film has a special optical property called surface plasmon resonance, which is affected by tiny amounts of surrounding materials, such devices have been used as biosensing applications.

Previous studies focused on manipulating in-plane two-dimensional EOT structures — for example, tuning the hole diameter, shape or distance between the holes. In addition, most were concerned with straight holes only. In Liu’s experiment, the EOT is mediated mainly by SPPs, which limits the sensitivity and figures of merit obtainable from such devices.

Sensor response to various chemicals. Courtesy of University of Illinois at Urbana-Champaign.
“Our current design employs 3-D subwavelength tapered periodic hole array plasmonic structure. In contrast to the SPP-mediated EOT, the proposed structure relies on localized surface plasmon mediated EOT,” Gartia said. “The advantage of [localized surface plasmons] is that the enhanced transmission at different wavelengths and with different dispersion properties can be tuned by controlling the size, shape and materials of the 3-D holes. The tapered geometry will funnel and adiabatically focus the photons onto the subwavelength plasmonic structure at the bottom, leading to large local electric field and enhancement of EOT.

"Secondly, the localized resonance supported by 3-D plasmonic structure will enable broadband tuning of optical transmission through controlling the shape, size and period of holes as well as the shape, size and period of metallic particles decorated at the side walls,” Gartia said. “In other words, we will have more controllability over tuning the resonance wavelengths of the sensor.”